1. [Technical Field of the Invention]
The present invention relates to a method for producing modified raw material milk for dairy products in which transglutaminase and a reducing agent are added to and caused to act on raw material milk for dairy products (i.e., milk as raw material for dairy products), such as raw milk or the like, whereby the milk proteins are cross-linked more effectively than before, in other words, a method for modifying milk as raw material for dairy products. By using the modified raw material milk produced according to the present invention, dairy products having improved physical properties, such as yogurt, cheese, powdered milk and the like having improved physical properties and provided with a favorable flavor and mouthfeel or texture, can be provided.
2. [Related Art]
In the production of dairy products, the value of the products depends largely on such physical properties as water-holding capacty, emulsion stability, viscosity, smoothness and the like. The manufacturers of dairy products have practiced a variety of devices to produce dairy products having further improved physical properties. For example, use of a variety of thickening polysaccharides for improving the mouthfeel and water-holding capacty of yogurt or ice cream has been widely known. Further, a variety of techniques such as use of a special lactic acid bacteria starter for reducing the separation of water from a yogurt (syneresis) (Japanese Patent Application Laid-Open No. 268874/1993), use of monoglycerides for obtaining a smooth processed cheese (Japanese Patent Application Laid-Open No. 105/1999), use of whey proteins for attaining the temperature stability of ice cream (Japanese Patent Application Laid-Open No. 135664/1997), and the like, have been reported.
Meanwhile, an attempt to improve the quality of a dairy product by using transglutaminase (Transglutaminase being abbreviated as xe2x80x9cTGxe2x80x9d hereinafter), which is an enzyme having the action of crosslinking proteins, has been reported. For example, it has been reported that the viscosity of a yogurt is increased by adding TG during the yogurt manufacturing process, whereby the separation of water from the yogurt (syneresis) is alleviated or reduced (Japanese Patent Application Laid-Open No. 197688/1994), that the yield of cheese curds is increased by using TG in the production process (Japanese Patent Application Laid-Open No. 173032/1996), and the like.
Such quality improvements with the use of TG have a variety of advantages from the industrial standpoint. Firstly, since even only an extremely small amount of TG is sufficient to exhibit the above effects, and at the same time, it acts directly on a food protein to exhibit the above effects, it has little adverse effect on mouthfeel. For example, when a thickening polysaccharide is added to a yogurt to improve its physical properties, even though such effects as an increase in viscosity, the prevention of separation of water from the yogurt or the like can be indeed attained, but the addition of the polysaccharide may not necessarily lead to an improvement in the overall quality of the yogurt due to the xe2x80x9cglueyxe2x80x9d mouthfeel of the thickening polysaccharide per se.
Secondly, in response to the customers"" needs for taking in so-called xe2x80x9cfood additivesxe2x80x9d as little as possible, use of such an enzyme has a natural feel and provides a high added value to commercial dairy and the like products. Incidentally, a natural microorganism-derived TG has already been commercialized and widely used in various food processings.
The reports which have heretofore been made about the reactivity of TG to milk proteins are mainly those about the studies of whey proteins, particularly xcex1-lactoalbumin and xcex2-lactoglobulin which are the constituents of the whey protein, or a condensed whey protein. The whey protein is known as a protein to which TG has low reactivity due to its structure in which it has Sxe2x80x94S bonds in the molecule (Fargemand et. al., J. Agric. Food. Chem. (1997) 45, 2,514-2,519, particularly p. 2,517, lines 41 to 53). For example, Traore and Meunier have reported that when Factor XIII (TG in blood) is introduced to act on the whey protein, crosslinking polymerization does not proceed in the absence of a reducing agent (J. Agric. Food Chem. (1992) 40, 399 to 402).
Further, Aboumahmoud and Savello (J. Dairy Sci. (1990) 73, 256 to 263) have reported that when xcex1-lactoglobulin or xcex2-lactoalbumin is crosslinked with the use of TG derived from guinea pig liver for the purpose of making a protein-based film, these whey proteins have to be preheated in the presence of a reducing agent at 85xc2x0 C. for 15 minutes.
Fargemand et al. (Food Hydrocolloids, (1997) 11, 19 to 25) have made a report about the reaction between a whey protein and a calcium-independent TG and additionally reported that the TG has the effect of increasing crosslinked polymer in the presence of dithiothreitol (DTT) or cysteine and the effect of increasing the crosslinked polymer to some extent even under the alkaline conditions.
Casein, which is the main protein of milk proteins, is already known as a protein to which TG has a high reactivity (Fargemand et al., Food Hydrocolloids (1997) vol. 11, no. 3, pp. 287 to 292). For example, Nio et al. have reported the crosslinking polymerization of xcex1S1-casein with a TG derived from guinea pig liver (Agrc. Biol. Chem. (1986), 50, 851 to 855), and Traore et al. have reported the crosslinking polymerization of purified caseins, particularly xcex2-casein and xcexa-casein, with a Factor XIIIa from human.
As compared therewith, few studies have been made on the reactivity of TG to the casein in cow milk. Only Nonaka et al. have made a report about the study in which the crosslinking polymerization and gelation of reduced skimmed milk powder with TG were compared with those of a caseinate with TG. In the report, it is stated that the casein in the reduced skimmed milk powder is inferior to the caseinate in terms of reactivity (J. Food. Sci., (1992), 57(5), 1214 to 1218).
Not a few studies have been made on the physical properties of a gel or dairy product derived from the cow milk on which TG has been acted. For example, Fargemand et al. have reported the influence of crosslinking of the casein in skimmed milk powder by TG on acidic gelation (Food Hydrocolloids (1997) vol. 11, no. 3, pp. 287 to 292), Lauber et al. have reported the crosslinking of casein by TG and the gel strength of a yogurt (Eur. Food Res. Technol., (2000), 210(5), 305 to 309), and Imm et al. have reported the gelation and water-holding capacty of the skimmed milk powder treated with TG (J. Food Sci., (2000), 65(2), 200 to 205). In addition, Lorenzen et al. have reported the properties of a yogurt made from the cow milk treated with TG, the physical properties of a whipped cream, and the formability of curd with rennet (Kiel. Milchwirtsch. Forschungsber. (1997), 49(3), 221 to 227).
As has been described above, in the crosslinking reaction of cow milk proteins with TG, attempts to decrease the required amount of TG or reaction time by improving the reactivity of TG to a milk protein, particularly a casein, are not yet been made. The reasons for this are, for example, because when evaluation was made on the function or the like of a gel, milk used as a raw material had had sufficient reactivity to observe the effects caused by the addition of TG and that the need for further improving the reactivity of a casein which is an effective substrate for TG had not been recognized, and the like.
Meanwhile, a reducing agent such as glutathione or the like is used in an enzyme reaction for the purpose of stabilizing an enzyme or improving reactivity to the enzyme. As described above, an example thereof is to improve the reactivity of a whey protein by treating the whey protein with a reducing agent such as DTT to reduce Sxe2x80x94S bonds.
As an example of improving the gel properties of food by using a reducing agent together with TG when TG was used on a food protein other than milk protein, it is known that the gel formability of frozen ground fish meat (surimi) can be improved by using TG, a protease inhibitor and a reducing agent in combination (S.-T. Jiang., et al., J. Food Sci. (2000), 65, 241 to 245). The reducing agent used in the above example is a sulfite. The reducing agent was, however, studied based on the amount which far exceeds the amount allowed to be added to food, and therefore has left a problem from the practical standpoint
Further, it is believed to be desirable to contain a milk protein and a thiol group-containing compound for modifying a food protein using TG and an oxidoreductase (Japanese Patent Application Laid-Open No. 161849/1999). In this case, the thiol group-containing compound only serves as a substrate for the oxidoreductase and is not an essential element. In addition, even if this stabilizes and retains the activity of TG, the effect of improving the reactivity of the food protein to TG has not been mentioned.
Although it has been already described above that use of TG in dairy products has a number of advantages, not many commodity products in which TG has been actually used are on the current market. The reasons for this are thought to be because the expected effects are not obtained in the actual production of dairy products using raw milk or because the actually obtained effects are not valuable from the industrial standpoint. In other words, even if some effects can be obtained by the addition of TG in the actual production of dairy products using raw milk, the effects are not so significant to be considered valuable from the industrial standpoint.
Under these circumstances, the present inventors have made intensive studies on the reactivities of raw milk and a variety of milks as raw material using raw milk to TG and methods of treating these milks with TG. As a result, they have found that milks as raw material having a low heat history such as raw milk, have a low reactivity to TG.
The present inventors have found that the milk is first preheated to improve the reactivity of the milk to TG, so that the crosslinking reaction of the raw milk by TG can be promoted, to solve such a problem.
For example, it has been confirmed that cow milk sterilized at a low temperature (sterilized at 63xc2x0 C. for 30 minutes) and cow milk sterilized at an ultra high temperature (sterilized at 130xc2x0 C. for 2 to 3 seconds) have different reactivity to TG, i.e., the latter has a higher reactivity to TG than the former. Further, it has been confirmed that when the former is heat-treated (heated to 90xc2x0 C.), the reactivity to TG is significantly improved.
Lorenzen et al. have also reported preheating raw material milk (at 95xc2x0 C. for 2 seconds) before the reaction with TG in the production of a yogurt using TG (Kieler Milchwirtschaftliche Forschungsberichte, (1999), 51(1): 89 to 97).
However, although preheating of the milk is simple and easy, a preheating step must be provided separately from the reaction with TG. This affects the production process of dairy products and also requires thermal energy and time. Further, since the denaturation of a milk protein by heat treatment is promoted, a loss of the flavor of raw milk and an adverse effect on the mouthfeel of a dairy product by heating are concerned. In addition, there is also the problem that preheating cannot be applied to dairy products such as cheese to which excessive heat treatment is undesirable.
[Problem to be Solved by the Invention]
It is an object of the present invention to solve the above problems and provide a method for producing a dairy product having improved physical properties by improving the reactivity of TG to raw material milk such as raw milk.
[Means for Solving the Problems]
The present inventors have made intensive studies to find a method in which the effects caused by use of TG can be obtained more effectively without preheating raw material milk having a low reactivity to TG even when TG is used (added) in a small amount and the reaction time is short. As a result, they have obtained the technical findings that the reactivity of TG to the raw material milk can be significantly improved by adding a reducing agent and TG to the raw material milk and allowing the mixture to react.
As a result of further studies, the effect of improving the reactivity has been observed when the amount in terms of concentration, of the reducing agent to be added to the raw material milk, for example, in the case of reduced glutathione, is at least 7xc3x9710xe2x88x925 g per 1 gram of the non-fat milk solid. This amount corresponds to 0.0006% by weight of the raw material milk (non-fat milk solid: 8.4%). Further, the present inventors have also made studies on a yeast extract containing glutathione in high concentration from the viewpoints of its usability for food, cost effectiveness, functionality and stability and have observed the effect of improving the reactivity when the yeast extract is added in an amount of at least 0.007% based on the raw material milk. The effect of this concentration on the taste of food is negligible, and this has verified that the present invention is a technology with extremely high practicality in that it can be practiced by using a natural substance.
The method of the present invention for producing modified raw material milk comprising using TG and a reducing agent in combination. The two materials may be added to raw material milk simultaneously, or one of them may be added prior to the other. However, it is preferable to add them and allow them to react simultaneously or treat with TG after the addition of the reducing agent.
That is, the present invention relates to modified raw material milk obtained by adding practical amounts of a reducing agent and TG to raw material milk and allowing them to react to crosslink the milk proteins contained in the raw material milk more efficiently, and a novel production method capable of improving the physical properties of a dairy product produced by using the modified raw material milk.